The present invention relates to an exhaust valve for an internal combustion engine, the exhaust valve comprising a valve disc having a first side and a second side, a valve stem extending from the first side of the valve disc, and an annular valve seat area located at the first side of the valve disc, which annular valve seat area is of a nickel-base alloy or a chromium-base alloy and has a valve seat surface.
Such an exhaust valve is disclosed in U.S. Pat. No. 6,295,731 where a valve blank has a valve disc of smaller diameter than the disc of the completed valve and excess material above the valve seat. The excess material is shaped as an isosceles-triangle-sectioned lobe on the valve disc covering the complete area of the valve seat. The valve blank is heated to a temperature of 530 to 600° C. and is forged in a die having the shape of the completed valve disc. During forging the isosceles-triangle-sectioned lobe is plastically deformed and the outer diameter of the valve disc is increased. The forging of the excess material thus causes plastic flow of all the material at the outer area of the valve disc.
EP 0 521 821 B1 discloses an exhaust valve comprising a valve disc having a first side provided with an extending valve stem and an annular valve seat area with a valve seat surface. The annular valve seat area is of a nickel-base alloy or a chromium-base alloy. Forging may be used to globally shape the valve seat area. Hot deformation is recommended in order to distribute carbide formations within the material in order to obtain improved corrosion resistance.
The book ‘Diesel engine combustion chamber materials for heavy fuel operation’ published in 1990 by The Institute of Marine Engineers, London, collected the experience gained from the whole industry and provided the general conclusion that the valve seat area of an exhaust valve must have high hardness.
WO 97/47862 discloses an exhaust valve disc having a base body of austenitic stainless steel and a valve seat area of a nickel-base material provided by welding or by a HIP process. The yield strength of the valve seat area may be increased by cold-working of the material, such as by rolling or forging the valve seat area.